Reduction of Contaminants in Soil and Water By Direct Electric Current - Electrokinetics for Petroleum and Environmental Engineers - page 50

reversal (Rojo et al, 2012) and the increasing potential gradient (Lei et al.
2012). Diffuse double layer (DDL) processes of clay have been suggested to
influence in situ conversion of heavy metal soil contaminants to potentially
less toxic forms when an external direct current is applied to the clay-elec-
trolyte system (Pamukcu et al, 2004, 2008; Brosky and Pamukcu, 2013).
The DDL shrinks, reducing the electrokinetic potential (i.e., zeta potential)
of the system in the event of lowered pH and/or increased ionic concentra-
tion of the pore fluid due to accumulation of charges in DDL (Israelachvili
and Adams, 1978). Therefore when the electrokinetic potential of a clay-
electrolyte system changes under varying pH or electrolyte concentration,
the system may become prone to spontaneous Faradaic reactions as it tries
to restore the equilibrium back. In electrokinetic treatment of clays, the
transient acid front movement from anode to cathode, and the regions of
low and high pH developing near the electrode sites can set the stage for
triggered Faradaic reactions resulting in beneficial transformation of the
contaminants.
In soils, all particles with active interfaces (i.e., Inner (IHP) and the
Outer Helmholtz Planes (OHP) (Hunter, 1981)), can be regarded to “act”
as electrode interfaces when a current is supplied to them, adding up to
a significantly large effective electrode surface area. Current flows across
these interfaces via two pathways: the Faradiac and non-Faradiac. When
an external electric field is applied to water saturated clay of high ionic
concentration in the pore fluid, given the incompatibility between the con-
ductivity of two conducting layers in the mixture: 1) the DDL of clay parti-
cles with low conductivity ( σ s ), and 2) the surrounding electrolyte solution
(bulk solution or pore fluid) with high conductivity ( σ b ), a large electrical
potential is induced across the DDL. This results in compression of the
DDL, resulting in higher charge density in this region. This is similar to
the effect when salt concentration of the bulk fluid is increased causing
the compression or shrinkage of DDL. The electric potential distribution
shifts back (i.e., towards the solid) and down, lowering the electrokinetic
potential at the shear plane (ς , zeta potential ) as shown in Figure 2.8a,b,c.
The potential distribution shift in this manner increases the intensity of
the electric field within the Stern layer (between IHP and OHP), while
decreasing the Capacitance, C of the DDL. The reduction in capacitance
of the diffuse layer can trigger electron transfer across the diffuse layer
towards the solution, giving rise to a Faradaic “cathodic” current (Bard and
Faulkner, 1980). The capacitance, C, of the double layer is directly related
to the bulk electrolyte concentration and the surface potential of the par-
ticle. The capacitance reaches a minimum at the point of zero charge (PZC)
and sharply increases on either side for the low electrolyte concentrations